1. Field of the Invention
The invention relates to a method of controlling an injection molding machine.
2. Description of Related Art
Control systems for injection molding machines must execute a wide variety of different control/regulation functions. Said functions include, for example, control and/or regulation of a hydraulic means adapted in particular for moving an injection molding tool and/or injection molding die. Further, the screw conveyor, for example, must be operated and moved. Moreover, exact regulation and control of pressures and temperatures are necessary.
Conventional control systems for injection molding machines comprise a central control unit which typically is a Programmable Logic Controller (PLC). A visualization means comprising an industrial computer (IPC) and a display is used for visualization of the program sequences. The control unit is connected via an input/output means with the injection molding machine. Using the PLC, the machine program is cyclically executed. According to the relevance of the individual regulation and control functions, cycles of different duration of normally 100 ms, 10 ms and 2 ms are performed. During a cyclical execution of machine program sequences, each sampling of a switch, sensors and the like can be generally repeated only within a specific time period.
The execution of cyclical machine program sequences has considerable drawbacks, for example when a change-over from injecting the injection molding material to the holding pressure is carried out. The change-over from injection to the holding pressure is normally performed in a path- or pressure-dependent manner. In the two change-over variants, when a predetermined analog value has been reached, a hydraulic pump or a servo valve is changed over to a new target value which is normally lower than the previous target value. During cyclical machine program sequences, such as those executed by a PLC, irregularities frequently occur. If a corresponding signal is sampled, for example within a time period of one millisecond at a velocity of 200 mm/sec., a maximum error of 0.2 mm may occur. In the case of pressure-dependent change-over, pressure gradients of 10,000 bar/sec. occur, which may result in inaccuracies of 10 bar. The magnitude of an error depends on the point of time during the cycle at which the sampling is carried out, since only at the end of the cycle a corresponding action is taken, such as change-over to the holding pressure. The errors thus have different magnitudes. Consequently, as compared with constant errors, such an error cannot be compensated for.
Examinations revealed that cyclical machine program sequences have the drawback that the cycle time is subject to variation. Variations of up to 10% may occur. Consequently, the occurring error cannot be compensated for.
For reducing the occurrence of such errors it is common practice to trigger an interrupt signal as soon as a corresponding signal is given. The interrupt signal interrupts the cycle, and the control is capable of responding immediately and changing over from injection to the holding pressure. Triggering of such an interrupt signal is however an intervention into the cyclical machine program sequence which is thus error-prone.
In particular, the control modes triggered by the interrupt signal must be communicated to the cycle. Further, the generation of interrupt signals and the interruption of the cyclical execution of the machine program sequence have the drawback that the machine program steps following the interrupt signal are postponed in terms of time. This may result in disturbances. In particular in the case of successive control signals within a machine program sequence, which correspond with each other and/or are founded on each other and, for example, control an operation, the interruption may lead to a considerable disturbance.
The cycle of an injection molding machine includes a lot of movements and actions. In this connection it is not only important that the individual movements and actions are executed in a time-stable manner, but the combination of movements and actions must be extremely time-stable either. Since an injection molding process is a thermodynamic process, an exact cycle time is decisive for the dwell time of the thermoplastic material in the dosing and injecting means. Only when this dwell time can always be exactly complied with, the manufactured product comprises the required constant material qualities. Due to the cyclical execution using conventional IPCs or PLCs, inaccuracies in terms of time occur during each movement and/or action and each transition from one movement/action to the next. In the course of an overall cycle, these inaccuracies may cancel each other out, but they may also add up. This may result in noticeably differing cycle times which may lie within a range of 200 ms.
Further, older control systems are known to use a control unit essentially made up of hardware components instead of an electronic control unit. Here, for example, the position of a screw conveyor is detected via a cam controller, and the pressure and quantity control is effected via a hardware. Such systems are however extremely inflexible non-programmable systems, such that flexibly usable injection molding machines cannot be equipped with such control systems.